1. Technical Field
The present invention relates, generally, to a gas assisted injection molding system. More specifically, the invention relates to a gas nozzle employed in such systems.
2. Description of the Related Art
Gas assisted plastic injection molding is a well established and commercially accepted method for providing plastic articles having a hollow interior. These hollow, plastic articles have numerous advantages, such as high strength, low weight, reduced plastic material cost and improved article appearance due to less shrinkage stress. A detailed discussion of the development of gas assisted injection molding technology is contained in U.S. Pat. No. 5,110,533 and incorporated herein by reference.
In gas assisted injection molding, the articles are produced by injecting molten resin into the mold cavity and injecting a quantity of pressurized gas into the resin to fill out the mold cavity and form a hollow portion in the resin. The gas is preferably an inert gas such as nitrogen. The gas pressure is maintained in the mold cavity and against the resin until the plastic has cooled sufficiently to be self supporting. Thereafter, the gas is vented, the mold is opened and the plastic article is removed from the cavity. One example of a gas assisted molding apparatus is disclosed in U.S. Pat. No. 5,639,405 issued Jun. 17, 1997 and incorporated herein by reference.
Generally speaking, there are two points of entry for gas in an injection molding environment: (1) at the injection molding machine nozzle; and (2) in the mold.
When the gas is injected through the same nozzle employed for injecting the plastic into the mold, the gas pressure must be relatively high because the gas bubble will not penetrate the plastic until the gas pressure is greater than the plastic injection pressure. In addition, any restriction such as at the gate will impede the bubble penetration requiring higher initial gas pressures to move the plastic to fill out the mold. However, when the pressure is too high, and once the bubble breaks through the gate, the gas will rocket through the cavity which is at a lower pressure to the end of the plastic flow front. If this occurs, the gas may escape the envelope of the plastic material unless there is extra resin in the cavity to resist this high pressure. Such elevated initial gas pressures at the plastic injection nozzle may wash away most of the plastic that is adjacent the gate including the material at the nominal wall. Gas injection at the plastic nozzle also requires complicated resin shut-off devices, valves and sealing members which ultimately wear out and are generally expensive.
On the other hand, gas may be injected directly into the mold at either the mold cavity (in-article) or at some point along the runner (in-runner). Where gas is injected directly into the mold cavity, the initial gas pressure at the beginning of the gas filling phase of the process can be much lower than that employed at the resin injection nozzle. The lower gas pressure will tend to complete the polymer fill at a velocity that is closer to the initial polymer fill velocity, thereby avoiding a gloss variation between initial polymer fill and gas pressure fill. Gas nozzles located in the runner are advantageous where the design of the part or structure of the mold does not lend itself to the in-article approach.
Numerous gas nozzles have been proposed in the related art to take advantage of the design and engineering advantage of mold cavity and runner gas injection. For example, stationary gas nozzles have been employed in the related art because such nozzles generally involve a reduction or elimination of any moving parts. Such stationary nozzles are simple and cost effective. However, stationary nozzles suffer from the disadvantage that they often become clogged with resin during the injection process and must be cleaned on a regular basis.
In order to overcome this problem, gas nozzle designers have incorporated resin check valves to block the flow of molten resin into the gas nozzle. Unfortunately, these resin check valves increase the cost and complexity of such nozzles.
Another solution proposed in the related art involves a hollow gas nozzle with an interior solid pin that has been relieved on a portion of one side to allow for a gas passage through the nozzle. Unfortunately, problems still exist with such nozzles of the related art. More specifically, these gas nozzles are typically mounted from the back side of the mold which is fixedly mounted to the platen of an injection molding press. If the mold is overshot during the injection process, as can frequently be the case, excessive injection pressure can clog the nozzle with resin. In this case, the mold must be removed from the platen and the gas nozzle disassembled and cleaned. Further, gas may exit such nozzles only through the top or terminal end thereof which is a limiting factor in the design of the part and the mold. Finally, larger parts often require a larger volume of gas flow and the relieved area of the gas nozzle does not accommodate this larger volume.
Additionally, the nozzles of the related art discussed above suffer from the disadvantage that once they are connected to a source of pressurized gas, such as when mounted in the mold or runner, they are not easily adjusted thereafter. This disadvantage is particularly apparent when the direction of the flow of gas from the nozzle is important. This often occurs when it is desirable to direct the flow of gas in a particular way into the mold and the gas nozzle includes an outlet which extends generally perpendicular to the main flow passage of the nozzle or out the side of the nozzle tip.
Thus, there is a need in the art for a simple, cost effective, efficient, stationary gas nozzle which includes no moving parts and which will not clog with resin during polymer fill or gas venting even after repeated shots of the mold. Further, there is a need in the art for such a nozzle wherein the direction of the flow of gas may be easily adjusted and wherein the tip of the nozzle may be quickly and easily changed without complicated maintenance or extended shut down time.